Sputtering is a process where a target, for example, tantalum or silicon, is bombarded with ions in a vacuum chamber. This bombardment causes atoms to be ejected from the surface of the target, which are then deposited as a thin film on a substrate. In the case of optically transparent films, a reactive gas, such as oxygen or nitrogen, may also be present. These reactive gases can then form oxide or nitride thin films on a substrate. In the design of optical multilayer coatings, it is very important to be able to control precisely the thickness of the deposited films.
The customary way of monitoring film thickness is to pass a light beam through the growing film and measure the changes in transmissivity or reflectivity mainly due to optical interference effects. With a knowledge of the relevant optical constants, the thickness of the film can be calculated. Unfortunately, in conventional magnetron sputtering the substrate must be placed close to the sputtering target in order to maximize the deposition rate and the thickness uniformity. This distance is typically in the order of 10 to 20 centimeters.
Because of the proximity of the substrate to the sputtering target, it is very difficult to perform any kind of optical monitoring except at an oblique angle of incidence. This causes the beam spot to spread over a large area and makes the beam very sensitive to wobble.
The only way of monitoring the growth of a film at near normal angles of incidence is to increase the target-to-substrate distance and spatially offset the substrate from the target so as to allow light to pass through the substrate. Unfortunately, moving the substrate away from the sputtering target significantly lowers the deposition rate, and more importantly can degrade the microstructure of the deposited film as the bombardment becomes less energetic.
It is theoretically possible to employ reflection-based optical monitoring at an oblique angle of incidence. Polarization effects, however, become important for angles of incidence greater than about 15 degrees, which makes this technique more difficult to implement.
The above limitations have made it impossible, or at least very difficult, to make certain coatings. For example, some quarter wave based coatings cannot be accurately deposited by sputtering without continuous optical monitoring.
Another requirement for optical monitoring is that the beam alignment be maintained with a high degree of precision. There may be considerable movement of the components of the sputtering chamber due to thermal expansion and pressure changes, and it is important that the beam strike the film and detector in the same location on the film during the entire deposition run to avoid systematic monitoring errors.
An object of the invention is to alleviate this problem.